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Airplane Takeoff, Find Thrust

  1. Feb 11, 2010 #1
    1. The problem statement, all variables and given/known data

    An Airbus A320 jetliner has a takeoff mass of 75,000 kg. It reaches its takeoff speed of 82 m/s in 35s.

    2. Relevant equations

    F=ma
    V/t=a

    3. The attempt at a solution

    I am completely lost on this question. I have seen numerous examples on the internet, with the exact problem, but no one has gotten the correct answer. After two hours and 20+ wrong answers I gave up and opted to see the answer. However, I would like to know how it was solved. Here is what I did that got me close to the right answer.

    Solved for acceleration indepentant of friction. Which was 82/35 giving me 2.343m/s^2.

    I plugged that in for f(thrust)=ma -> (75,000)(2.343) = 175,725 N.

    This is too low and doesn't involve rolling friction. From a table in my book I figured rolling friction was 0.020 for rubber on concrete. I then solved for the friction.

    m=75,000
    g=9.80
    f=0.020

    F=ma -> F=(75,000)(9.80)(0.020) = 14,700N

    Since the thrust will have to include overcoming friction the whole time, I add those two values and get 190,425 N.

    This is the only way I can come close to the actual answer. I've done it a bunch of different ways, none were within 20k of the answer.

    How should one actually solve this?

    The answer given is 190,000N.
     
  2. jcsd
  3. Feb 11, 2010 #2
    !!!
    [tex] F =/= ma [/tex]
    [tex] F_{net}=ma[/tex]

    So expanding Fnet gives us
    [tex] F_t -F_f = ma [/tex]
    Plugging everything in gives us a thrust of 190 000 N.
     
  4. Feb 11, 2010 #3
    Did I have the value for friction right?

    Because using my values and your expansion, I get the same answer I had before...
     
  5. Feb 11, 2010 #4
    I would imagine the answer is using two sig figs since all numbers are given with two sig figs.
     
  6. Feb 11, 2010 #5
    That makes sense, thanks for the help. I'll try the problem again later and see if I remember it.
     
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